(a) Technical Field
The present invention relates to an apparatus and a method for controlling a motor-driven power steering system, more particularly, to an apparatus and a method that are capable of inhibiting a change in steering feeling which may be caused by abrasion and degradation of components in the motor-driven power steering system, and maintaining initial steering feeling.
(b) Description of the Related Art
In general, known types of power assistance steering systems for reducing a steering effort of a driver include: a hydraulic power steering (HPS) system, which assists a steering effort of a driver by using hydraulic pressure produced by a hydraulic pump; and a motor-driven power steering (MDPS) system (hereinafter, referred to as ‘MDPS’), which assists a steering effort of a driver by using driving torque of an electric motor.
In the case of the MDPS, output of the electric motor for steering assistance may be controlled in accordance with a traveling condition of a vehicle when a steering assistance function is carried out as the driver manipulates a steering wheel, thereby providing improved steering performance and steering feeling as compared to the hydraulic power steering system.
Therefore, the MDPS, which is capable of changing and controlling steering auxiliary power produced by motor output in accordance with the traveling condition, has been widely applied to vehicles that are recently released on the market.
The MDPS may include sensors such as a steering angle sensor which detects a steering angle (column input angle) according to manipulation of the steering wheel, a torque sensor which detects steering torque (steering wheel torque and column torque) inputted through the steering wheel, a vehicle speed sensor which detects a vehicle speed, a wheel speed sensor, an engine rotational speed sensor, a yaw rate sensor, a controller (MDPS ECU), and a steering motor (MDPS motor).
The controller receives, from the sensors, steering input information such as a steering angle, a steering angular velocity (an angular velocity value obtained from a signal differentiated from a steering angle signal), and steering torque, and vehicle state information (a vehicle speed, a wheel speed, an engine rotational speed, a yaw rate, etc.) in order to drive the steering motor and control output of the steering motor.
In this case, the controller controls driving power (motor output) of the steering motor in accordance with a vehicle speed so as to produce auxiliary torque adjusted for steering assistance, and the controller increases the motor output at a low speed so that the driver may manipulate a light steering wheel, and decreases the motor output at a high speed so that the driver may manipulate a heavy steering wheel, thereby ensuring traveling stability of the vehicle.
If the steering wheel is too light when the vehicle travels at a high speed, a dangerous situation may occur even by a slight manipulation of the steering wheel, and driving stability becomes poor. Therefore, by changing steering assistance characteristics in accordance with a vehicle speed so as to assist the driver so that the driver may manipulate a heavier steering wheel when the vehicle travels straight at a high speed, a stable steering wheel manipulation may be carried out.
Typically, the output of the steering motor, which assists a driver steering effort, is obtained as the controller controls a motor electric current to be applied to the steering motor (auxiliary control, electric current amount control).
In this case, the controller basically provides the steering motor with an electric current amount corresponding to an output value (steering system output torque, i.e., steering motor output torque) determined based on the steering input information and the vehicle state information after tuning and calculating the electric current amount, and the controller generates force for assisting a driver steering effort by driving the motor.
In the steering system, constituent elements for transmitting the driver steering effort applied through the steering wheel and steering auxiliary power generated in accordance with the steering effort, include: a steering column which is installed at a lower side of the steering wheel, a gearbox which converts rotational force being transmitted from the steering column into straight force and changes a direction of a tire, and a universal joint which transmits rotational force transmitted to the steering column to the gearbox.
Here, the gearbox includes a pinion gear which receives rotational force from the universal joint, and a rack bar which has a rack meshing with the pinion gear, and the rack bar is moved straight leftward and rightward by the rack when the pinion gear is rotated.
In this case, force, which is generated as the rack bar moves straight leftward and rightward, is transmitted to the tire via a tie rod and a ball joint, thereby changing a direction of the tire.
Meanwhile, mechanical abrasion or durability degradation of components of the motor-driven power steering system may occur as the components are used over a long period of time, such that output of the motor-driven power steering system may be changed.
If the output of the motor-driven power steering system is changed, there is a problem in that steering feeling felt by the driver varies, and therefore, there is a need for a method of compensating for output against abrasion or degradation of components.
FIG. 1 (RELATED ART) is a view for explaining a problem in the related art, and illustrates durability test data for a rack-and-pinion gear. The data shows degrees of friction of the rack-and-pinion gear with respect to cumulative values of the number of durability test steering, that is, the number of left/right full-turn steering of the steering wheel for a durability test when the steering wheel is repeatedly and fully turned leftward and rightward in a state in which vehicle wheels are stopped.
As illustrated in FIG. 1, it can be seen that as a cumulative value of the number of durability test steering is increased, a degree of friction of the rack-and-pinion gear is decreased because of the occurrence of abrasion.
Because of abrasion and durability degradation of components such as the steering motor or bushings in the steering system, which occur when the components have been used over a long period of time, in addition to abrasion of the rack-and-pinion gear, output of the steering system may be changed due to a reduction in friction between the components even under the same condition, which changes steering feeling felt by the driver.
FIG. 2 (RELATED ART) is another view for explaining a problem in the related art, and illustrates curves indicating a steering angle and steering torque under a particular vehicle speed condition, in which (a) is a hysteresis curve indicating steering torque when the motor-driven power steering system is mounted in the vehicle and begins to initially start, and (b) is a hysteresis curve indicating steering torque which has been changed as the motor-driven power steering system is operated for a predetermined period of time and durability degradation occurs.
In a case in which the durability degradation occurs as illustrated in FIG. 2, the driver feels light and loosened steering feeling (i.e., a steering effort is small) when the driver manipulates the steering wheel, and particularly, the steering effort greatly becomes small at an on-center portion, that is, a neutral position of the steering wheel.
Accordingly, there is a need for control logic that allows the driver to feel initial steering feeling before abrasion or durability degradation of the components occur even though the abrasion or the durability degradation of the components occur.
After assembling the components, a degree of mechanical friction may be changed due to a manufacturing variation at the time of manufacturing the components, and thus, there is also a need for a method of compensating for the change in degree of mechanical friction and allowing the driver to feel a predetermined level of steering feeling.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.